Many manufacturing processes require continuous measurements of the consistency of a slurry being processed and a prime example is in the paper making art where the consistency of the aqueous fiber suspension must be monitored and controlled to ensure a uniform final product Electro-optical apparatus for this purpose is known and prior art examples may be found in U.S. Pat. Nos. 3,665,201, 3,990,795 and 4,040,743. The present invention relates generally to the broad class of apparatus shown in these patents and constitutes a significant improvement thereover in that the consistency readings are substantially independent of both ligth absorption by the liquid component of the slurry and the brightness of the suspended particulate matter.